This application is based on application No. Hei 10-22905 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to image reading apparatus and, more particularly, to an image reading apparatus for reading an image from a developed film.
2. Description of the Prior Art
To read an image from a developed film by using an image reading apparatus compatible with, e.g., an advanced photo system (APS), there has conventionally been performed the process of drawing the film out of a cartridge by means of a winding member, converting the image recorded thereon to an electric image signal via an image pickup element such as a CCD, and reproducing the image on a display or the like.
Such a conventional image reading apparatus has used generally a CCD line sensor as an image pickup element to read an image by scanning a film, which is effective in obtaining an image with high resolution. However, an excessively long time is required by the conventional image reading apparatus to read an image because of the film scanning time is required. If a CCD area sensor is used instead as the image pickup element, an image can be read momentarily.
In order for the image reading apparatus using the area sensor to achieve high resolution comparable to that achieved by a system using the line sensor for scanning the film, the number of CCD pixels should be increased significantly. To accomplish this, there can be used a method in which the number of pixels per unit area is increased or a method in which the CCD size is increased. Normally, there is used the method in which the number of pixels per unit area is increased. In accordance with the method, however, an acceptable error range of back focus required is reduced in an image-forming optical system in the image reading apparatus since the optical system is generally composed of a lens combination for reducing image size.
In the case where a single lens barrel holds an optical system composed of lenses having positive, negative, positive, and positive refractive powers which are arranged in this order for image size reduction, a variation in back focus caused by temperature becomes unidirectional. In other words, an elongated distance between each adjacent two of the lenses and an elongated distance between any of the lenses and the CCD cooperate to move a focal point in one direction, so that it is impossible to perform temperature compensation. This causes the problem that a variation in back focus caused by temperature cannot be compensated by a depth of focus.
For temperature compensation in an optical system, there has been devised a conventional structure in which a lens barrel for holding is divided into three members to cause the extension of the middle member to occur in the direction opposite to the extension of two other members, as disclosed in Japanese Unexamined Patent Publication No. Hei 8-160276.
If the above mentioned image-forming optical system is applied to such conventional structure for temperature compensation, what results is a structure in which all lenses are held by a first lens barrel having a low thermal expansion coefficient, the end portion of the first lens barrel closer to the CCD is held by a second (middle) lens barrel having a high thermal expansion coefficient, and the end portion of the second lens barrel closer to the front end of the lenses is held by the third lens barrel having a low thermal expansion coefficient.
In this case, the middle lens barrel covers the first lens barrel holding all the lenses so that the diameter of the middle lens barrel is increased in proportion to a maximum lens diameter, which limits the scaling down of the apparatus. Further scaling down can be achieved if the apparatus is similarly constituted by regarding the CCD as a lens. In that case, however, temperature compensation is not performed properly since the CCD itself is a heating element and has a temperature different from the ambient temperature of the lens.
It is therefore an object of the present invention to overcome the foregoing problems and provide an image reading apparatus which allows correction of a back focal shift caused by thermal deformation in the image-forming optical system.
To attain the object, one aspect of the present invention is embodied in an image reading apparatus comprising: an illuminator for illuminating an original image on a film: an image-forming optical system for forming an image from the original image illuminated by the illuminator; and an image pickup element for converting the image formed by the image-forming optical system into an electric image signal, the image-forming optical system being an optical system for image size reduction composed of an anterior group having a high error sensitivity and a posterior group having a low error sensitivity when viewed from the illuminator side, the anterior group being held by a first lens barrel composed of a material having a low thermal expansion coefficient, the posterior group being held by a second lens barrel composed of a material having a high thermal expansion coefficient, the image pickup element being held by a third lens barrel composed of a material having a low thermal expansion coefficient, the first and third lens barrels being held by the second lens barrel.
The optical system is composed of four lenses having positive, negative, positive, and positive refractive powers, respectively. If the four lenses are designated at G1, G2, G3, and G4, respectively, the anterior group is composed of the lenses G1 to G3 and the posterior group is composed of the lens G4. Alternatively, the anterior group is composed of the lenses G1 and G2 and the posterior group is composed of the lenses G3 and G4.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings, which illustrate specific embodiments of the invention.